Research and development on magneto-optical disks have been intensified as being rewritable optical disks, and some of the magneto-optical disks have been already practically used as external memory designed for computers.
In the magneto-optical disk, a magnetic thin film with perpendicular magnetization is used as a recording medium, and light is used in recording and reproducing. Thus, compared with a floppy disk or a hard disk wherein a magnetic thin film with in-plane magnetization is used, the magneto-optical disk has a larger recording capacity.
Recently, a magneto-optical recording medium which includes a recording layer composed of a magnetic layer having a multi-layer structure, which enables a reproduction of a recording bit with a size significantly smaller than the size of the light spot, i.e., a magneto-optical recording medium which enables a reproduction of high resolution has been proposed.
For example, Japanese Examined Patent Publication No. 81717/1993 (Tokukohei 5-81717) discloses a magneto-optical recording medium which includes a recording layer for recording information magneto-optically and a readout layer has in-plane magnetization at room temperature, and in which a transition occurs from in-plane magnetization to perpendicular magnetization when the temperature thereof is raised.
The magneto-optical recording medium enables reproduction of high resolution for the reasons explained with reference to FIG. 22 through FIG. 24.
As shown in FIG. 22, the magneto-optical disk is mainly composed of a substrate 101 whereon a transparent dielectric layer 102, a readout layer 103, a recording layer 104, a protective layer 105 and an overcoat layer 106 are laminated in this order.
As shown in the magnetic phase diagram of FIG. 23, a composition range where the rear-earth transition metal alloy used in the readout layer 103 has a perpendicular magnetization (shown by A in the figure) is extremely narrow. This is because perpendicular magnetization appears only in the vicinity of a compensating composition (shown by P in the figure) where the magnetic moment of the rare-earth metal and the magnetic moment of the transition metal balance with one another.
The respective magnetic moments of the rear-earth metal and the transition metal have mutually different temperature dependencies. Specifically, the magnetic moment of the transition metal is greater than that of the rear-earth metal at high temperature. Thus, the composition of alloy is set such that the content of the rear-earth metal is greater than that in the compensating composition at room temperature so that the alloy does not have perpendicular magnetization at room temperature but has in-plane magnetization. When a light beam is projected, as the temperature of the portion irradiated with the light beam is raised, the magnetic moment of the transition metal becomes relatively greater until it balances with that of the rear-earth metal, thereby having perpendicular magnetization.
FIG. 24(a) through FIG. 24(d) show one example of the hysteresis characteristics of the readout layer 103. In each figure, x-axis indicates an external magnetic field (Hex) to be applied perpendicularly onto the surface of the readout layer 103, and y-axis indicates polar Kerr rotation angle (.theta.K) when a light beam is incident perpendicularly on the surface of the readout layer 103.
FIG. 24(a) shows hysteresis characteristic of the readout layer 103 in a temperature range of room temperature--T.sub.1, the transfer layer 103 having the composition shown by P in the magnetic phase diagram of FIG. 23. FIG. 24(b) through FIG. 24(d) respectively show hysteresis characteristics in temperature ranges of T.sub.1 -T.sub.2 ; T.sub.2 -T.sub.3 ; and T.sub.3 -Curie temperature T.sub.c.
In the temperature range of T.sub.1 -T.sub.3, the readout layer 103 shows such a hysteresis characteristic that an abruptly rising of Kerr rotation angle appears with respect to the external magnetic field. In other temperature ranges, however, the polar Kerr rotation angle without the application of the external magnetic field is substantially zero.
When the rear-earth transition metal having the described characteristics is applied to the readout layer, a higher density of the magneto-optical disk can be achieved. Namely, the reproduction of the recording bit with a size smaller than the size of the light spot is enabled for the reasons presented below.
As shown in FIG. 22, when reproducing, a reproduction-use light beam 107 is projected onto the readout layer 103 from the side of the substrate 101 so as to form a light spot 109. Here, on the recording layer 104, information is recorded in the magnetization direction shown in the figure. In the light spot 109 formed on the readout layer 103, the central portion is heated to the higher temperature than the peripheral portion. More specifically, since the reproduction-use light beam 107 is converged to a diffraction limit by the objective lens 105, the light intensity distribution shows a Gaussian distribution, and thus the temperature distribution of the portion subject to reproduction of the magneto-optical disk also like a Gaussian distribution. In the case where the reproduction-use light beam 107 is set such that the temperature of the central portion of the irradiated area in the readout layer 103 is raised above T.sub.1 in FIG. 23 and the temperature of the peripheral portion is not raised above T.sub.1, only the portion having a temperature rise above T.sub.1 is subject to reproduction. Thus, the reproduction of a recording bit with a size smaller than the diameter of the light spot 109 is permitted, thereby achieving a significant improvement in the recording density.
When a transition occurs in the portion having a temperature above T.sub.1 from in-plane magnetization to perpendicular magnetization (from the state shown in FIG. 24(a) to the state shown in FIG. 24(b) or the state shown in FIG. 24(c)), by the exchange coupling force exerted between the readout layer 103 and the recording layer 104, the magnetization in the recording layer 104 is copied to the readout layer 103. On the other hand, the temperature in the portion corresponding to the vicinity of the light spot 109 is below T.sub.1, and thus in-plane magnetization is maintained (FIG. 24(a)). As a result, with respect to the light beam irradiated in a direction perpendicular to the film surface, the polar Kerr rotation effect is not shown.
As described, when a transition occurs from in-plane magnetization to perpendicular magnetization in the portion having a temperature rise, only the central portion of the light spot 109 shows the polar Kerr rotation effect, and the information recorded on the recording layer 104 is reproduced based on the reflected light from the irradiated area.
When the light spot 109 is shifted (in practice, the magneto-optical disk is rotated) so as to reproduce the next recording bit, the temperature of the previous bit drops below T.sub.1 and the transition occurs from perpendicular magnetization to in-plane magnetization. Accordingly, the polar Kerr effect is no longer shown in the spot having the temperature drop. Therefore, information is no longer reproduced from the spot having the temperature drop and thus interference by signals from the adjoining bits, which causes noise, can be eliminated.
As described, the magneto-optical disk permits a reproduction of a recording bit with a size smaller than the diameter of the light beam 7 without being affected by the adjoining recording bits, thereby achieving a significant improvement in the recording density.
In the embodiment of the high density magneto-optical disk, the inventors of the present invention disclose the properties and effects in the "Proceedings of Magneto-optical Recording International Symposium' 92, J. Magn. Soc. Jpn., Vol. 17, Supplement No. S1 (1993), pp. 201-204, "Super Resolution Readout of Magneto-Optical Disk with an In-plane Magnetization Layer".
In the publication, in the recording medium having the arrangement shown in FIG. 22, GdFeCo is used as the readout layer 103, and DyFeCo is used as the recording layer 104. GdFeCo has such properties that the composition range that shows perpendicular magnetization is extremely narrow (shown by A in FIG. 23), and in which a transition suddenly occurs from in-plane magnetization to perpendicular magnetization. Therefore, GdFeCo is a suitable material for the recording medium of high intensity. FIG. 25 shows a temperature dependency of the polar Kerr rotation angle measured from the side of the readout layer of the recording medium. Here, the threshold temperature at which a transition occurs from in-plane magnetization to perpendicular magnetization is around 100.degree. C. At temperature below 100.degree. C., since in-plane magnetization is shown, the polar Kerr rotation angle is extremely small. In the vicinity of 100.degree. C., since a transition suddenly occurs from in-plane magnetization to perpendicular magnetization, the polar Kerr rotation angle suddenly increases.
The described threshold temperature is an important factor that determines the reproduction laser power in reproducing a signal by the laser beam. FIG. 26 is a graph which shows the relationship between the reproducing laser power and the amplitude of the reproducing signal in the recording medium. As shown in the graph, the signal amplitude suddenly increases with an increase in the reproducing laser power, and is maximized at around 2 mW-2.3 mW. From the above-mentioned principle, it can be seen that a signal can be achieved only after the area having a temperature above 100.degree. C. appears in the reproducing light spot. As can be seen from the graph, when the threshold temperature is set to 100.degree. C., a desirable reproducing power is in a range of 2 mW-2.3 mW. When the threshold temperature is above 100.degree. C., the reproducing power is required. However, when the reproducing power becomes too high, unwanted recording may occur by the reproducing power, and the information may be destroyed. On the other hand, when the threshold temperature is set below 100.degree. C., information can be reproduced with a lower reproducing power. However, when the threshold temperature is set too low, i.e., around 40.degree. C., the environmental temperature for reproducing is set to 40.degree. C., the temperature in the entire spot becomes above 40.degree. C., and the reproduction of high resolution cannot be achieved.
As described, in the magneto-optical recording medium, it is important to control the threshold temperature at which a transition occurs from in-plane magnetization to perpendicular magnetization.
In addition, when reproducing, in an area having a temperature above the threshold temperature, it is required that the magnetization in the recording layer is surely copied to the readout layer (in a direction perpendicular to the film surface, upward or downward). In other words, the magnetization direction (upward or downward) in the recording layer is copied to the portion subject to perpendicular magnetization of the readout layer.
The above conditions can be achieved by controlling each composition based on the magnetic interaction between the magnetic layers (the readout layer and the recording layer).
In order to stabilize the recorded information, it is required that the coercive force at room temperature is large, and also required to have such a Curie temperature that an extremely large laser power is not required for recording.
Although the objective is not to achieve a reproduction of high resolution by the magnetic supper resolution, the magneto-optical medium having a recording film of multi-layer structure has been proposed. For example, Japanese Examined Patent Publication No. 35371/1990 (Tokukohei 35371-2) discloses a magneto-optical recording medium having a magnetic layer of double-layer structure, which prevents the reproducing signal quality (S/N ratio) from being lowered even when the recording power is lowered, and as an example of the recording medium, which stabilizes the recorded information with respect to the external magnetic field. The magneto-optical recording medium is composed of the magnetic layer having a high Curie temperature of above 200.degree. C., made of amorphous alloy including Gd-Fe, or Gd-Co and small coercive force (the magnetic layer corresponding to the readout layer) and magnetic layer made of Tb-Fe or Dy-Fe having a low Curie temperature, i.e., in a range of 200.degree. C. to 50.degree. C. and large coercive force, and the magnetic layer of large coercive force and the magnetic layer of small coercive force are exchange coupled.
As one effect of the configuration, since the magnetic layer having small coercive force and high Curie temperature that is exchange coupled with the magnetic layer of large coercive force, information can be read from the magnetic layer of small coercive force, and in the reproducing operation, a desirable S/N ratio can be achieved.
In the previously explained magneto-optical recording medium, in order to achieve a still higher recording density, it is required to improve the quality of the reproducing signal when reproducing. In the magneto-optical disk, from the reproduction principle, a larger the Kerr rotation angle is, a greater the reproducing signal is.
Therefore, by making the polar Kerr rotation angle of the readout layer (hereinafter referred to as .theta.K) larger, a signal quality can be improved.
Like GdFeCo, the RE-TM (rear-earth-transition metal) alloy thin film using FeCo as a transition metal, a greater .theta.k can be achieved by increasing the proportion (density) of Co in FeCo.
However, by laminating the readout layer onto the recording layer having an increased Co density, for example, in the recording medium which enables a reproduction of high resolution, the threshold temperature at which a transition occurs from in-plane magnetization to perpendicular magnetization is shifted to the side of high temperature, that is a main factor for enabling a reproducing operation of high resolution is shifted to the side of high temperature, and the laser power required for reproduction increases. Not only the problem that the life of the laser becomes shorter, if the threshold temperature is too high, the problem arises also in that the reproducing power becomes excessive, which may destroy the recorded information when reproducing.
Moreover, in the case of the Japanese Examined Patent Publication No. 35371/1990 (Tokukohei 2-35371), when the readout layer (corresponding to the magnetic layer of small coercive force) subject to the varying composition on the recording layer (corresponding to the magnetic layer of large coercive force), the exchange copying force varies. The exchange coupling force which is an important factor in determining the desirability of the reproducing operation from the recording medium, from which recorded information is readout from the magnetic layer of small coercive force by the exchange coupling force from the magnetic layer of large coercive force (see column 5, line 43--column 6, line 1). When the exchange coupling force varies, the problem of incomplete reproducing or recording operation arises.
In the case of using other recording medium having a plurality of magnetic layers on or from which a recording or reproducing operation is performed using the magnetic interaction between the magnetic layers (including an exchange coupling force), the above-mentioned problems presented are equally presented.